The present invention relates to an electric melting furnace used to vitrify waste by utilizing Joule heat, and more particularly to a waste vitrifying electric melting furance provided with electrodes at least in the center of the interior of a melting tank and on the side walls therein.
This melting furnace is used for the vitrification of various kinds of industrial waste, especially, for the vitrification of high radioactive level liquid waste.
In a conventional waste vitrifying electric melting furnace, a furnace body and a melting tank are made of refractory bricks, and one or a plurality of pairs of electrodes consisting of a heat-resistant metal (alloy) or a metallic oxide are provided on the side walls in the melting tank.
An electric current is applied to molten glass in the melting tank via the electrodes by utilizing the electric conductivity of the molten glass, to generate Joule heat which heats the molten glass. When radioactive liquid waste and glass additive are supplied from the upper portion of the melting furnace to the surface of the molten glass in the furnace, they are heated with the molten glass to turn to molten glass through a temperature increasing step, an evaporation step and a calcination step.
The soundness of the refractory bricks constituting the melting furnace and the material of the electrodes depend upon the temperature of the molten glass which contacts these materials. Accordingly, it is desirable that the temperature distribution of the molten glass in the furnace be kept uniform. In order to meet this requirement, various configurations of the electrodes have been adopted for trial in accordance with the shape of the furnace and the quantity of the waste to be treated.
In recent years, the treatment of high radioactive level liquid waste involves a problem that an abnormal temperature rise occurs in the bottom portion of the furnace and a decrease of the waste treatment capacity occurs. This is ascribable to the following. The elements of the platinum group, such as ruthenium, palladium and rhodium contained in the high radioactive level liquid waste form electrically conductive substances which are difficult to dissolve in the molten glass and are deposited at the bottom of the furnace. Since the electric current flowing between the electrodes is concentrated on the conductive deposit, the Joule heat generated in the molten glass is not supplied sufficiently to the surface of the molten glass above the conductive deposit.
For example, it has been reported that, when conductive substances having an electric resistance 7/8 times as high as that of a regular glass, and consisting of elements of the platinum group deposited 5 cm deep in a furnace bottom, the electric characteristics of the molten glass changed to cause the glass throughput capacity to decrease from 30 kg/hr to 20 kg/hr (Horst Wiese, "Industrial Vitrification of High Level Liquid Waste with The PAMERA Plant in Belgium" in and Hazardous Waste Management SPECTRUM '88 held in Pasco, Washington, U.S.A. on Sept. 11-15, 1988, page 76).
It has also been reported that the results of several experiments conducted by using a laboratory-scale melting furnace having an inclined furnace bottom showed that a furnace bottom having an angle of inclination of 45.degree. was effective for the discharging of the conductive deposit consisting of elements of the platinum group, and further reported that this effect was confirmed by the results of experiments conducted by using a full-size melting furnace having the furnace bottom of the same angle of inclination (Shin-ichiro Torata, "Development of Glass Melter for PNC Tokai Vitrification Facility" in the proceedings of the same meeting as above-described, page 82).
However, in a conventional melting furnace, it is anticipated that a small quantity of conductive substances will be deposited on the bottom portion or inclined surfaces of the glass melting furnace in spite of the inclined furnace bottom. In such a case, it is possible that the above-mentioned phenomena, i.e. the electric abnormality or a decrease in the waste treatment capacity, occur. Such inconveniences occur because the electric current flowing between the side electrodes constitutes the supply source of heating current in the conventional melting furnace, and the electric current necessarily flows from one side electrode to the other via the conductive deposit on one inclined surface, the conductive deposit on the furnace bottom and the conductive deposit on the other inclined surface. Consequently, abnormal heat is generated in the vicinity of the furnace bottom, and the heating current density at the upper portion of the molten glass decreases. This causes a decrease in the waste treatment capacity to occur.